In certain types of nuclear reactors, the nuclear fuel is contained in fuel rods. The fuel rods are grouped in fuel bundles, within which the fuel rods are equidistantly positioned from each other in a spaced array. The array itself is supported between an upper and a lower tie plate. A number of these fuel bundles are combined to form the nuclear fuel assembly.
An arrangement of the type described is shown in U.S. Pat. No. 4,022,661, which is assigned to the assignee of the present invention. As shown, each fuel rod is resiliently supported between the tie plates by virtue of helical springs surrounding the elongated, reduced-diameter shanks of the upper end plugs of the fuel rods. This arrangement allows longitudinal expansion of the individual fuel rods. It also insures that the fuel rods are firmly seated in the lower tie plate to dampen vibration as pressurized liquid coolant flows upward through the fuel bundle to remove heat from the fuel rods. Typically, the lowest coil of each helical spring is of reduced diameter to provide a friction fit on the end plug shank. This inhibits "pop off" of the springs when the upper tie plate is removed. Further, it inhibits the spring from falling off during manipulation of a fuel rod outside the array. Not withstanding these precautions, springs are occasionally lost from the shanks of the end plugs. Poor visibility and other factors may prevent such a condition from being discovered before the upper tie plate is replaced or first installed.
Once the fuel bundle is assembled, the upper tie plate obstructs the view of an observer, particularly with respect to those springs which are mounted on end plugs located near the center of the assembly. Thus, any visual inspection for the presence of these springs is precluded without removing the upper tie plate. Since the act of removing the tie plate may itself cause springs to be lost, as well as being a cumbersome and time-consuming procedure, there currently exists no satisfactory way for reliably detecting the absence of helical springs on the shanks of fuel rod end plugs in the assembled fuel bundle.